The present invention relates to tetrafluoroethylene thermoprocessable copolymers under the form of granules having a well defined morphology and controlled size (hereinafter called xe2x80x9cmicrospheresxe2x80x9d), having an improved flowability, to be used in various applications, such for example rotomouldidng, powder coating, etc.
In particular, the invention relates to TFE thermoprocessable copolymer microspheres having a substantially spherical shape for at least 95% by weight, the microsphere size being in the range 25 xcexcm and 2 mm, the bulk density being in the range 0.5 and 1.1 g/cm3. The fluoropolymer microspheres of the present invention, having a substantially spherical shape and a particularly high bulk density, can advantageously be used in various applications such as for example powder coating, rotomoulding, flame spraying and as inert support in gas-chromatography columns.
It is known that from the polymerization reactor of fluorinated monomers a fluorinated polymer latex is obtained which is subsequently subjected to the conventional coagulation technique. A coagulum forms under the form of fine powder, having an average particle size lower than 35 xcexcm. The particles have a low bulk density, in the range 0.2-0.5 g/cm3, and a substantially irregular, i.e. not spherical, shape such for example shown in the photo of FIG. 4 (by scanning electronic microscope (SEM)). The coagulum particles have therefore few industrial applications. For example in applications as the powder coating, an average particle size in the range 25-80 xcexcm is required. This means that only one portion of the coagulum is obtainable by sieving. However, the sieving of particles having a so small size is difficult since clogging of the sieves takes place. On the other hand by using the coagulum as such, a non uniform coating is obtained having different properties.
Furthermore, due to the very small size, these coagulum particles cannot be used in rotomoulding and rotolining applications. Indeed, for these applications they must necessarily be extruded and supplied to the users under the form of milled or unmilled pellets, having sizes from 100 xcexcm to 1,000 xcexcm. However, due to their very low bulk density and their irregular shape, the coagulum particles flow with difficulty during the extrusion process. This poor flowability often implies these two drawbacks: 1) a low extruder productivity and 2) block in some extruder section, with consequent bad functioning of the extruder.
It is furthermore known that in gas-chromatography columns, PTFE as inert support under the form of particles having an average size in the range 200-500 xcexcm can be used. This porous PTFE support is mainly formed by particles with an irregular, not spherical, shape, having a low bulk density, in the range 0.3-0.5 g/cm3. Due to the low bulk density, it is necessary to carefully use this support, since it easily tends to compress itself and to become a compact solid rather than to remain porous, when it is sbjected to stress forces during the use and during the gas-chromatography column packing. For this reason, during the column packing phase, a cooling under 0xc2x0 C. is necessary to disperse the static load and harden this support. Besides, this inert support, because of the non spherical shape of the particles, has an empty space degree which considerably changes with the loss of pressure, wherefore the columns result particularly sensible to any variation of the loss of pressure which can lead to a breaking of the support continuity. The obtained packing, having a low bulk density, shows furthermore unsatisfactory mechanical and electric properties negatively affecting the column efficiency.
Generally it can be stated that the coagulum particles deriving from the conventional coagulation techniques, having a very low bulk density, show the drawback to easily disperse in the working environment due to their extreme lightness with consequent loss of useful product in the application phase and problems of environmental pollution for the operators. Another drawback resides in that they show an irregular or xe2x80x9cdendriticxe2x80x9d structure, which implies a low flowing capability and therefore transfer difficulty for clogging problems. This poor flowing capability limits the effectiveness of their use in all the applications where free-flowing properties are required, for example for the above applications.
The need was thefore felt to have available for the above mentioned applications of powder coating, rotomoulding, inert support for gas-chromatography columns, fluoropolymer microspheres having a regular substantially spheric morphology, such as to confer improved free-flowing properties in application phase, and having besides a higher bulk density, which implies improved mechanical resistance, smaller volume occupied in application phase, lower dispersion in the working environment.
An object of the present invention are therefore TFE thermoprocessable copolymer microspheres having a substantially spherical shape for at least 95% by weight, the average size of the microspheres being in the range 25 xcexcm-2 mm, the bulk density being in the range 0.5-1.1 g/cm3, preferably 0.55-1.0 g/cm3.
The substantial sphericity of the microspheres is shown in the photo of FIG. 3 ((SEM)).
For TFE thermoprocessable copolymers, polymers obtained by TFE polymerization with one or more monomers containing at least one ethylene type unsaturation are meant.
Among the TFE comonomer the fluorinated are in particular mentioned:
C3-C8 perfluorolefins, such as hexafluoropropene (HFP);
C2-C8 hydrogenated fluoroolefins, such as vinyl fluoride (VF), vinylidene fluoride (VDF), trifluoroethylene, hexafluoroisobutene, perfluoroalkylethylene CH2xe2x95x90CHxe2x80x94Rf, wherein Rf is a C1-C6 perfluoroalkyl;
C2-C8 chloro- and/or bromo- and/or iodo-fluoroolefins, such as chlorotrifluoroethylene (CTFE);
CF2xe2x95x90CFORf (per)fluoroalkylvinylethers (PAVE), wherein Rf is a C1-C6 (per)fluoroalkyl, for example CF3, C2F5, C3F7;
CF2xe2x95x90CFOX (per)fluoro-oxyalkylvinylethers, wherein X is: a C1-C12 alkyl, or a C1-C12 oxyalkyl, or a C1-C12 (per)fluoro-oxyalkyl having one or more ether groups, for example perfluoro-2-propoxy-propyl;
fluorodioxoles, preferably perfluorodioxoles;
non conjugated dienes of the type:
CF2xe2x95x90CFOCF2CF2CFxe2x95x90CF2,
CFX1xe2x95x90CX2OCX3X4OCX2xe2x95x90CX1F
wherein X1 and X2, equal to or different from each other, are F, Cl or H; X3 and X4, equal to or different from each other, are F or CF3, which during the polymerization cyclopolymerize.
Also hydrogenated olefins, preferably in addition to the above mentioned comonomers can be mentioned. Examples of hydrogenated olefins are ethylene, propylene, butene and isobutene.
Generally, for the semicrystalline thermoprocessable copolymers, the comonomer amount in the copolymer is between about 0.05 and 18% by weight, preferably 0.5 and 10% by weight, and it depends on the type of comonomer.
The TFE copolymers can also be amorphous-vitrous depending on the comonomer. They can be obtained for example by using as comonomers the dioxoles or the monomers which cyclize during the polymerization. In this case the comonomer amount can be much higher, generally higher than 20% by weight.
Therefore the TFE copolymers of the present invention can be semicrystalline and amorphous-vitrous, provided they are thermoprocessable. The skilled is easily capable to determine, by routine tests, the comonomer amount to have a thermoprocessable polymer, i.e. thermomouldable.
Examples of thermoprocessable copolymers are:
FEP copolymers, i.e. tetrafluoroethylene-hexafluoropropene (TFE/HFP) copolymers, described for example in U.S. Pat. No. 2,946,763;
FEP copolymers modified with a third monomer, for example TFE/HFP/PEVE (perfluoroethylvinylether) copolymers described in EP 759,446 and U.S. Pat. No. 5,677,404;
TFE/HFP/PMVE (perfluoromethylvinylether) copolymers described in U.S. Pat. No. 5,688,885;
TFE/HFP/PPVE (perfluoropropylvinylether) copolymers described in U.S. Pat. No. 4,029,868.
In the FEP copolymers the HFP amount is about 5-10% by moles, while the perfluoroalkylvinylether amount in FEPs modified with vinylether is between about 0.2 and 3% by weight.
Other thermoprocessable copolymers preferred in the present invention are:
TFE/PPVE copolymers commecially called PFA, described in U.S. Pat. No. 3,635,926;
copolymers TFE/PMVE/fluorinated monomer selected from the above mentioned ones, preferably PPVE, wherein the PMVE amount ranges from 0.5 to 13% by weight, the fluorinated monomer amount ranges from 0.5 to 3% by weight. See for example U.S. Pat. No. 5,463,006; the terpolymer TFE/PMVE/PPVE is commercially called MFA.
copolymers TFE/PMVE/fluorinated dioxole wherein PMVE is in the range 0.5%-13% by weight, the fluorinated dioxole is in the range 0.05%-3% by weight. Such copolymers are described in U.S. Pat. No. 5,498,682. As fluorinated dioxole, 2,2,4-trifluoro-5-trifluoro-methoxy-1,3-dioxole (TTD) is preferably used.
The polymerization of the thermoprocessable copolymers of the invention can be carried out in emulsion according to methods known in the prior art, preferably according to the processes described in U.S. Pat. No. 5,498,682 and U.S. Pat. No. 5,463,006. Polymerization latexes are obtained from which the microspheres of the invention are obtained by the process reported hereunder.